1. Field of the Invention
The present invention relates to optical modules equipped with a wavelength locker.
2. Description of Related Art
Wavelength division multiplexing (WDM) communication, which multiplexes light of different wavelengths to increase the transmission capacity, is used in optical transmission systems. WDM communication makes it possible to increase the transmission capacity per signal light wavelength band used by narrowing the signal light wavelength interval of multiplexed signal light. Currently, in WDM communication, 200 GHz, 100 GHz and 50 GHz signal light wavelength intervals have been put into practical use, and signal light wavelength intervals of 25 GHz and 12.5 GHz are sought in order to increase the transmission capacity.
In WDM communication, since multiple signal lights are transmitted with a specific signal light wavelength interval, it is necessary to have adequate isolation from signal light of adjacent wavelengths, and as the signal light wavelength interval becomes narrower, higher precision control of the output wavelength of the signal light source becomes necessary. In WDM communication systems, waveguide lockers are used, which inhibit wavelength fluctuation due to chronological fluctuation or fluctuation of surrounding temperature of the laser diode (LD) light source and lock the output light with high precision to a specific wavelength (for instance, Japanese Unexamined Patent Application Publication 2001-291928).
With the development of multiple wavelength WDM communication systems, from the viewpoint of convenience of system operation and inventory management, tunable LDs capable of outputting multiple WDM signal light wavelengths in a single unit have come to be used as the LD modules constituting the light source element. Etalon filters that allow adequate wavelength detection output to be obtained for multiple WDM signal light wavelengths and have wavelength periodicity are generally used in wavelength lockers of LD modules using tunable LDs.
FIG. 8 is an LD module having a wavelength locker function based on the prior art. LD module 3C, shown in top view in FIG. 8(a) and in side view in FIG. 8(b), comprises an LD element 10, LD carrier 11, forward first lens 12A, rearward lens 12B, beam splitter (BS) 30A, photodiodes (PDs) 20A and 20B, PD carriers 21A and 21B, etalon filter 31C, mount carrier 40, thermoelectric cooler (TEC) 41, optical isolator 50 and forward second lens 19, and is connected to optical fiber 60.
FIG. 9 is a plot of electrical current values, obtained by inputting LD output light into an etalon filter and monitoring the transmitted light with a PD, plotted against the change in LD output wavelength, and shows the transmittance characteristic of the etalon filter. An etalon filter is a filter that makes use of the fact that incoming light undergoes multiple interference by parallel plates or parallel membranes of interval L provided in a medium.
As shown in FIG. 9, the filter characteristics of an etalon filter are periodic, and its period of change (FSR: free spectral range) is expressed using the refractive index n of the medium, the interval L between the parallel plates or parallel membranes, and the constant c as:FSR=c/2*n*L  (1)Namely, due to the periodicity of the transmittance characteristic of an etalon filter, if the input light intensity is the same, the monitor current values based on light with a wavelength shifted by an integer multiple of the FSR will be equal: for instance, in FIG. 9, for λ1, λ2, λ3 and λ4, each with a wavelength shifted by FSR, the difference in input wavelength cannot be distinguished based on the monitor current.
The filter characteristic and peak wavelength of an etalon filter shift depending on the angle of incidence of incident light. FIG. 10 shows the peak wavelength shift of an etalon filter due to changes in angle of incidence. If the angle of incidence of the input light signal entering an etalon filter is increased, the peak wavelength will shift to the shorter wavelength side, and the entire filter characteristics shown in FIG. 9 will shift to the shorter wavelength side as well.
In FIG. 8, the LD element 10 installed on LD carrier 11 outputs light forward (to the right in the figure) and rearward (to the left in the figure). The forward output light of LD element 10 is turned into parallel light by the forward first lens 12A, passes through optical isolator 50, is converged by the forward second lens 19, and is outputted to the optical fiber 60. The optical isolator 50 transmits light outputted by the forward first lens 12A and blocks light reflected by the second lens 19, thereby preventing output intensity fluctuation and output wavelength fluctuation of the LD element 10 due to reflected light.
Meanwhile, rearward output light is collimated by the rearward lens 12B and inputted into BS 30A. Rearward output light split by BS 30A is inputted into PD 20A mounted on PD carrier 21A to monitor the output light intensity. Rearward output light that has passed through BS 30A passes through etalon filter 31A and is inputted into PD 20B mounted on PD carrier 21B.
Thermoelectric cooler (TEC) 41 is an element that monitors the temperature of the LD element, for instance an element that adjusts the temperature of LD element 10 to keep it constant based on the results of monitoring by thermistor resistance or the like.
The monitored value of PD 20B is a value that reflects the output light intensity of LD element 10 and the transmittance characteristic of the etalon filter 31A, but by finding the quotient value by dividing the monitored value of PD 20B by the monitored value of PD 20A, the output fluctuations of LD element 10 are cancelled out, and an output intensity of the etalon filter 31A that corresponds to the output wavelength of LD element 10 can be obtained. That is, the quotient value obtained by dividing the monitored value of PD 20B by the monitored value of PD 20A has the characteristic shown in FIG. 9 in relation to the wavelength change of LD output light, regardless of the output light intensity of LD element 10, so changes in wavelength of the output light of LD element 10 can be detected based on changes in the quotient value.
As shown in FIG. 9, near the peak and near the bottom of the etalon filter transmission characteristic, the output of the etalon filter will not fluctuate much even if the input wavelength fluctuates, so minute changes in wavelength of the LD element cannot be detected based on the aforementioned quotient value. In LD modules having a wavelength lock function, in order to lock the output wavelength of the LD element with good precision, the sloped part of the characteristic is normally used, where the transmittance of the etalon filter varies greatly in response to wavelength fluctuation.
In order to monitor minute output wavelength fluctuations of an LD element, as shown in FIG. 10, the wavelength characteristic of the etalon filter is shifted by adjusting the angle of incidence, so as to make the target output wavelength of the LD element lie on the sloped part of the etalon filter characteristic.
The output light wavelength of the light source LD used in a WDM communication system is set up for each signal light wavelength interval of WDM communication systems, so the value used for FSR of the etalon used in the LD wavelength locker is the signal light wavelength interval when using either the left or right slope of the filter, and twice the signal light wavelength interval when using both the left and right slope of the filter.
As described above, with LD modules having a wavelength lock function, output wavelength changes of the LD element are detected based on monitor current changes due to the transmission characteristic of the etalon filter, controlling the output wavelength of the LD element 10 and fixing it at the target wavelength.
Since the transmission characteristic of the etalon filter used in the wavelength locker of FIG. 8 is periodic, light of a wavelength shifted by the FSR of the etalon filter cannot be distinguished based on the output transmitted by the etalon filter. That is, when wavelength locking is carried out based on a specific quotient value, there are multiple wavelength-lockable wavelengths of light.
Therefore, to achieve output of light of the target wavelength when the LD is started up, at the initial startup, it is necessary to first start up the wavelength locker in the vicinity of the lock wavelength (rough adjustment), and then perform wavelength locking based on the output of the wavelength locker unit, i.e. the monitored values of PD 20A and PD 20B (fine adjustment). Generally, the rough adjustment range of a wavelength locker using one etalon filter as shown FIG. 8 is at or below the FSR of the etalon filter.
Wavelength lockers are known that use multiple etalon filters in order to widen the rough adjustment range of the wavelength locker (e.g., Japanese Unexamined Patent Application Publication 2003-185502). FIG. 11 is a drawing that shows a conventional wavelength locker using multiple etalon filters, which has a configuration whereby light for monitoring the intensity of the output light of LD element 10 is split off by BS 30A, after that light for monitoring the light transmitted by etalon filter 31B used for rough adjustment is further split off by BS 30B, and the light transmitted by etalon filter 31A used for fine adjustment is monitored.
With the wavelength locker shown in FIG. 11, for instance by making the FSR of the etalon filter used for rough adjustment several times the signal light wavelength interval, fine adjustment of the output light wavelength of the LD element 10 can be performed over a broad range without going through a rough adjustment process at the time of initial startup.
With wavelength lockers using one etalon filter, in order to lock the output wavelength of the LD with good precision, it is necessary to use the sloped part of the characteristic where the transmittance of the etalon filter varies greatly in response to wavelength variation. While the filter characteristic of an etalon filter can be shifted by adjusting the etalon filter's slope angle, passive alignment performed while monitoring the passthrough output during assembly is essential for this, and the number of assembly processes increases.
Furthermore, as shown in FIG. 11, while it is possible to perform fine adjustment of the output wavelength of the LD over a broad range, without going through a rough adjustment process at initial startup, by using multiple etalon filters, for rough adjustment and for fine adjustment, this requires an additional etalon filter, BS and PD, and area is needed for installing the elements, so the overall volume of the LD module equipped with the wavelength locker increases. Furthermore, if an additional BS is used, the light splitting parts increase, so the number of assembly processes increases.